First-pass regex test: establish positive and negative cases

Start with a harmless sample that should match and a near-identical one that should not. Run both under the target engine and flags, then inspect match spans and capture groups. Add empty input and a longer malformed string if the expression will process user-controlled data. If the first pass behaves unexpectedly, fix the rule before applying it to a large dataset. A small test proves the intended boundary, not universal correctness.